/Engine/Source/Runtime/Core/Public/Misc/DateTime.h
A leap year is a year containing one additional day in order to keep the calendar synchronized with the astronomical year. All years divisible by 4, but not divisible by 100 - except if they are also divisible by 400 - are leap years.
true if the year is a leap year, false otherwise.
static bool IsLeapYear ( int32 Year )